Natural gas hydrates (NGH or clathrate hydrates of natural gases) form when water and certain gas molecules are brought together under suitable conditions of relatively high pressure and low temperature. Under these conditions, the ‘host’ water molecules will form a cage or lattice structure capturing a “guest” gas molecule inside. Large quantities of gas are closely packed together by this mechanism. For example, a cubic meter of methane hydrate contains 0.8 cubic meters of water and typically 164 but up to 172 cubic meters of methane gas. While the most common naturally occurring clathrate on earth is methane hydrate, other gases also form hydrates including hydrocarbon gases such as ethane and propane as well as non-hydrocarbon gases such as CO2 and H2S.
NGH occur naturally and are widely found in sediments associated with deep permafrost in Arctic and alpine environments and in continental margins at water depths generally greater than 500 meters (1600 feet) at mid to low latitudes and greater than 150-200 meters (500-650 feet) at high latitudes. The thickness of the hydrate stability zone varies with temperature, pressure, composition of the hydrate-forming gas, underlying geologic conditions, water depth, and other factors.
Worldwide estimates of the natural gas potential of methane hydrates approach 700,000 trillion cubic feet—a staggeringly large figure compared to the 5,500 trillion cubic feet that make up the world's currently proven gas reserves.
Most of the methane hydrate research to date has focused on basic research as well as detection and characterization of hydrate deposits. Extraction methods that are commercially viable and environmentally acceptable are still at an early stage of development. Developing a safe and cost effective method of producing methane hydrate remains a significant technical and economic challenge for the development of hydrate deposits.
Hydrate dissociation is a strongly endothermic process (i.e., in order to take place, the hydrate must draw in heat from the surrounding environment). The amount of heat available from surrounding geologic strata is often limited and the rate of heat flow is often slow. Initial thinking for hydrate production had been to provide dedicated external heat sources (for example steam boilers) to inject heat (for example hot water or steam) into the hydrate reservoir in order to provide sources of heat to support the endothermic dissociation process and provide consequently higher production rates of hydrocarbons. This is commonly called thermal stimulation. Economic analyses at the time were based on the cost of generating the steam or hot water in dedicated machinery, and showed this technique to be uneconomic. Research since that time has turned to production of hydrate reservoirs using depressurization (with endothermic heat provided by the earth itself). This gives understandably lower production rates than those obtainable by direct heating (thermal stimulation) because heat inflow is subject to the aforementioned geologic limits.